why would you want the screen to stay on during a call? it only turns off when your head is covering it up, meaning next to your ear. It saves battery and prevents accidental button presses. Having the screen stay on will contribute to heating up the device and battery drainage during long calls. It comes back on as soon as you pull it away from your head..............
you can try rhodium settings tool. it was made for the euro/gsm touch pro2, but the screen off feature may still works (look at the screen shot at the top right). i use it to disable push internet for real.
http://forum.xda-developers.com/showthread.php?t=549135